This invention relates to a method and system for controlling the start of a thermal power plant.
A thermal power plant has various systems such as a boiler system, a turbine system and a generator system. To start the plant, every system must made ready to operate. In the boiler system, for example, a fuel supply system is operated under the control of a fuel supply controller, a burner is then ignited, and the temperature and pressure of the water flowing through water-wall tube of the boiler are gradually raised. In the turbine system, a condenser is increasingly evacuated to such extent that superheated steam may be led into a turbine. When the steam is heated to such a temperature that so-called mismatch temperature, i.e. a difference between the temperature of the steam at the outlet of the first-stage steam chamber of the turbine and the temperature of an inner metal wall of the first-stage steam chamber, falls into a permissible range, the steam is supplied to the turbine. Then, the turbine starts rotating. The speed of the turbine is elevated until the turbine provides a rated output.
Hitherto, the temperature and pressure of superheated steam has been regulated by controlling the temperature and pressure of the water flowing through water-wall tube of the boiler in accordance with only the condition of heat-exchanging in a superheater and the start condition of the boiler, regardless the operation factors of the turbine. The steam in the boiler is gradually heated as heat-exchanging is effected by the superheater, no matter whether the power plant is started in so-called "boiler hot" mode or so-called "turbine cold" mode. In some cases the temperature of the superheated steam is so high that the mismatch temperature falls out of said permissible range. This is because the steam can be heated in a short time to such temperature that the mismatch temperature falls within the permissible range in case the power plant is started in boiler hot mode and the boiler is thus hot from the beginning. And this is because the temperature of steam which makes the mismatch temperature fall within the permissible range is low in case the power plant is started in turbine cold mode. If the temperature of the superheated steam is too high, it must be lowered until the mismatch temperature falls into the permisslbe range. This means that it takes a long time to start the power plant. If superheated steam of an excessively high temperature is supplied to the turbine, the rotor and other metal parts of the turbine will be heated so abruptly that an extremely large thermal stress is applied on the rotor. This would reduce the life of the turbine. Thus it is necessary to lessen the thermal stress on the rotor. To achieve this various measures are taken until the rotor rotates fast enough to provide a rated output. For example, the speed of turbine is adjusted or the load on the turbine is regulated.
As mentioned above, in the conventional method for controlling the start of a thermal power plant it takes a long time to start the plant since the start of the plant is not controlled according to the start mode, the boiler hot mode or the turbine cold mode.
Indeed with the conventional power supply system it does not matter if the start of the thermal power plant is delayed by some time. Recently, an emergency shut-down may take place often in nuclear power plants which are being built in increasing numbers. It is therefore increasingly demanded that thermal power plants be started as soon as possible in case an emergency shut-down occurs in the nuclear power plants.